Modern Vespa

Hello all
Still have some problem with my new Vespa 125 GT (2006). When I drive the temperature goes very high and then drops suddenly.
What the issues can be?
Thermostat under the reservoir tank? Thermostat on the engine?
Is the water pump?
How can I test the to see if they do what they supposed to.
Any help would be great.
Thanks

I'd say normal. The temperature rises until the thermostat opens and allows the water to flow through the radiator.

any numbers for this? and what happens when you continue riding for a bit longer? does it stabilise?

I don't think there are any numbers on the temp. gauge.

It almost reaches the red spot and then it drops. I have turned the bike off at that moment and when I restarted (after few minutes) the temperature was down.
It may be the temperature valve doesn't not opens time. What do you think?

I'd say normal but if worried replace the thermostat. My Kawasaki does something similar but if I don't look until I am actually going somewhere I don't notice.

I think this is not normal.
What the temperature should do is rise until not too far below red and then stay there. When the engine is heating up the thermostat will gradually open and keep the temperature constant.

As you describe it temperature goes up very high and then suddenly drops.
That sounds like a thermostat that is hampering. It needs a too high temperature to open and then it opens suddenly completely.

I would replace the thermostat.

But as background:
- Was it always like that? Or was temperature some time ago stable?
- Did this problem show up from one day to the next, or did temperature gradually get more unstable?
- Do you experience any other issues with the cooling? Too low coolant level?

If it is the thermostat I expect you would gradually experience more instability in water temperature. These things normally do not fail from one day to the next and if they do they would stay or closed all the time, overheating the engine, or stay open, and then the temperature would be too low all the time.

Never once, I have come across a failed thermostat in the closed position. They do fail (especially in very hot environment, or where the owner just use normal water), but they always fail in a mid or completely open position (by design)...

In my opinion, the OP has a faulty water temperature sensor. You can easily check the sensor with hot water and a Multimeter for resistance.